The present invention relates to the detection of incipient faults in dynamoelectric machines and especially large hydrogen-cooled machines such as turbine generators.
Various fault or abnormal conditions may occur in dynamoelectric machines such as partial or complete insulation failures, partial or complete breakage of conductors, local overheating or metallic parts of the machine such as the stator core, and others. If such conditions are not detected until a complete failure or other serious damage actually occurs, the machine may be badly damaged by excessive temperatures or by arcing or flashovers which may occur, and which may result in complete or catastrophic failure of the machine. At the very least, the occurrence of a serious fault may require extensive repairs such as partial or complete rewinding or rebuilding of parts of the machine, resulting in an extended shutdown of the machine and heavy expense.
It is desirable therefore to detect incipient faults before they can develop into complete failures, so that the trouble can be remedied before it causes serious damage. Most faults of this kind are associated with overheating and abnormal temperature rise has been utilized to indicate the presence of a fault or other problem. Temperature detectors in the stator slots or large machines have been used for many years to detect overtemperature but such detectors respond only to overheating of the conductors themselves such as may be caused by excessive conductor currents or by a failure of the ventilation system.
More recently, incipient thermal fault detectors have come into use which monitor the changes in concentration of particulate matter in the circulating hydrogen atmosphere of the machine. These systems are based on the fact that most organic insulating materials used in large generators give off particulate matter when heated and an increase in the concentration of particulates is thus a signal that overheating is occurring. This type of system, however, has several disadvantages. Most organic insulating materials exhibit particulation signals when the temperature reaches approximately 200.degree. C. The typical hydrogen gas temperature in a generator during operation, however, is approximately 60.degree. C. In order for the insulation to reach the necessary surface temperature, therefore, the temperature of the conductor itself must become very high before enough heat is conducted through the thick, high-voltage insulation. It has been shown by tests, for example, that copper conductors with usual insulation must reach a temperature of 600.degree. C before particulation occurs at the surface of the insulation. Fault detectors responsive to lower conductor temperatures would, therefore, be highly desirable. Fault detection by particulate concentration, furthermore, indicates only overheating of organic components, such as the surface layers of conductor insulation, and does not provide any direct indication of conductor temperatures or of overheated components other than organic insulation, such as metallic parts of the generator including the stator core. Such detectors are responsive only to temperature and do not meet the need for detecting arcing and corona faults which cause only very localized heating of the metal parts such as conductors.
Another disadvantage of the use of organic particulation for detecting thermal faults is the doubtful particulation life of organic materials at constant temperature. Tests indicate that the temperature at which particulation occurs increases with particulation time, and that particulation ceases to occur after some time period when the material is held at a constant temperature. Sustained and reproducible particulation signals are thus difficult to achieve under conditions of actual use. It has been attempted to overcome this problem by means of sacrificial coatings on the surface of organic insulation which would have lower particulation temperatures than the usually used materials. Such coatings, however, in addition to increasing the cost, usually possess only a limited particulation time at constant temperature.